The present invention relates to an active material for a hydrogen storage alloy electrode such as the negative electrode of a nickel-metal hydride storage battery, and to a method for producing the active material.
Recently, the nickel-metal hydride storage battery comprising, for the negative electrode, a hydrogen storage alloy capable of reversibly absorbing therein and desorbing therefrom hydrogen has been noted as a secondary battery of high energy density and long cycle life. With increased performance and diversification of portable appliances and development of new types of electric vehicles and hybrid cars which use the secondary battery as their power source, production of nickel-metal hydride storage battery has been increased in place of other conventionally used secondary batteries such as nickel-cadmium storage battery.
In the use for electric vehicles for example, however, there is a desire for a battery which can yield a high output allowing repeated charge/discharge cycles for about as long as a decade. The conventional nickel-metal hydride storage battery is not satisfactory in cycle life for such uses.
For improving cycle life of the battery, a variety of surface treatments for the hydrogen storage alloy and use of various additives to the negative electrode have been proposed as the techniques for inhibiting corrosion of hydrogen storage alloy.
For example, in the Japanese Laid-Open Patent Publication No. Sho 61-168866, it is suggested that the hydrogen storage alloy of the negative electrode is imparted corrosion resistance by plating the hydrogen storage alloy particle with Cu and making it into a microcapsule to give a long cycle life to the battery.
It is also suggested in the Japanese Laid-Open Patent Publications No. Hei 4-245165 and No. Hei 7-94176 that the negative electrode is added an alloy containing iron or iron compound as additives.
It is also suggested in the Japanese Laid-Open Patent Publication No. Hei 6-21576 that oxidation of the negative electrode is inhibited by incorporating yttrium in the negative electrode.
The above-mentioned prior art which microcapsulates the hydrogen storage alloy particle, however, is not fit for mass production because of high production cost and small capacity density of the obtained negative electrode.
The prior art which uses the additive such as iron compound for the negative electrode simply mixes the additive with electrode materials and is not intended to uniformly deposit the additive on the surface of alloy particle. With this technique, therefore, the deposition of iron on the alloy particle surface is not uniform and there occur iron-poor areas and iron-rich areas in the surface. Moreover, since iron has a poor solubility in alkaline solution, the deposited amount of iron on the alloy particle surface is small even if iron can be deposited after being released from the additive to the alkaline solution. This results in failure of formation of the surface layer comprising iron compound and the obtained inhibiting effect on corrosion is only small.
Despite a variety of alternatives including surface treatments for the hydrogen storage alloy particle, such as alkali treatment as suggested in the Japanese Laid-Open Patent Publication No. Sho 61-176063 and acid treatment :as suggested in the Japanese Laid-Open Patent Publication No. Hei 4-179055 which primarily aim at enhancing initial activation of the battery by increasing conductivity in the alloy particle surface portion, and use of additives to the negative electrode, such as addition of Co as suggested in the Japanese Laid-Open Patent Publication No. Hei 1-197965 and addition of Fe as suggested in the Japanese Laid-Open Patent Publication No. Hei 5-266887, hydrogen storage alloy electrode having a satisfactory cycle life has not been obtained.
In view of the above-mentioned drawbacks of the prior art, the object of the present invention is to provide an active material for a hydrogen storage alloy electrode to obtain A nickel-metal hydride storage battery having a longer cycle life than any conventional nickel-metal hydride storage battery.
The present invention is directed to an active material for a hydrogen storage alloy electrode, having A core alloy of a hydrogen storage alloy and a surface layer comprising iron compound formed on the surface of the core alloy.
In other words, the present invention is related to an active material for a hydrogen storage alloy electrode having a core alloy and a surface layer formed on the surface of the core alloy, the core alloy comprising a hydrogen storage alloy and the surface layer comprising iron compound.
In a preferred mode of the present invention, the iron compound is an iron oxide or iron hydroxide.
In another preferred mode of the present invention, an iron content ratio in the surface layer is 5 to 40 mol % of all metal content in the surface layer. In this case, it is further preferred that the core alloy contains iron at a lower content ratio to all metal contained in the core alloy than the iron content ratio it the surface layer to all metal contained in the surface layer.
It is also preferred that the core alloy is represented by the general formula: MmNiaFebMc, where Mm is a Misch metal or a mixture of rare earth elements, M is at least one selected from the group consisting of Mn, Al, Cu and Co, 0.05xe2x89xa6bxe2x89xa60.8 and 5.0xe2x89xa6a+b+cxe2x89xa65.5.
In still another preferred mode of the present invention, a crystal exists between the core alloy and the surface layer, the crystal comprising at least one selected from the group consisting of metallic nickel, metallic cobalt, nickel oxide and cobalt oxide.
It is also desirable that the core alloy contains Co and has A magnetic susceptibility of 0.3+Axc3x976.06 emu/g (emu/g=(4xcfx80)2xc3x9710xe2x88x9210 Hm2/kg) or more, where A is percent by weight of Co in the core alloy.
The present invention is also directed to a method for producing an active material for a hydrogen storage alloy electrode comprising the step of mechanically mixing a core alloy of a hydrogen storage alloy with an iron compound having a mean particle size of one-tenth or less the mean particle size of the core alloy to form a surface layer comprising iron compound on the surface of the core alloy.
In another mode of the method of the present invention for producing an active material for a hydrogen storage alloy electrode, the method comprises the steps of immersing a core alloy of a hydrogen storage alloy in an aqueous solution containing an inorganic acid salt of iron (a salt of inorganic acid and iron) dissolved therein, adding an aqueous alkaline solution to the aqueous solution containing the core alloy to form a surface layer comprising iron compound on the surface of the core alloy, and then washing the core alloy with the surface layer to remove alkali.
In still another mode of the method of the present invention for producing an active material for a hydrogen storage alloy electrode, the method comprises the steps of immersing a core alloy of a hydrogen storage alloy containing iron into an aqueous alkaline solution to form a surface layer comprising iron compound on the surface of the core alloy, and then washing the core alloy with the surface layer to remove alkali.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description.